The present invention relates to cementing pipe within a wellbore. More particularly, the present invention relates to selectively releasing wiper plugs contained within enclosed launching assemblies for cementing casing, subsea casing strings and casing liners in wells.
Pipe used to case wellbores is cemented into the wellbore to anchor the well pipe and isolate differently pressured zones penetrated by the wellbore. Pipe used for this purpose is generally referred to as xe2x80x9ccasing.xe2x80x9d The cementing step is initiated by pumping a cement slurry down into the casing from the well surface. The cement slurry flows out from the bottom of the casing and returns upwardly toward the surface in the annulus formed between the casing and the surrounding wellbore.
In the cementing process, the fluid normally used in the drilling of the wellbore, referred to herein generally as xe2x80x9cdrilling fluid,xe2x80x9d is displaced from the casing ahead of the cement slurry pumped into the casing. When a sufficient volume of the cement slurry has been pumped into the well pipe, drilling fluid is used to displace the cement from the well pipe to prevent the pipe from being obstructed by the cured cement.
The drilling fluid and cement slurry are separated during the displacements with appropriate liquid spacers, or more preferably, with sliding wiper plugs that seal along the inside of the well pipe, wiping the inside of the pipe and isolating the cement slurry from the drilling fluid. When using wiper plugs to separate the drilling fluid and cement, the cement slurry is pumped behind a first wiper plug to push the plug through the casing, forcing the drilling fluid in the casing to flow ahead of the plug. The drilling fluid displaced from the bottom of the casing flows upwardly through the annulus and returns toward the well surface.
When a sufficient volume of cement has been pumped behind the first wiper plug, a second wiper plug is positioned in the casing and drilling fluid is pumped into the casing behind the second plug to push the cement slurry through the casing. A flow passage in the first plug opens when it reaches the casing bottom to permit the cement slurry to flow through and past the plug, out the casing bottom. Once the first wiper seal has been opened and its seal terminated, the continued advance of the second plug through the casing displaces the cement slurry past the first plug, around the end of the casing, and up into the annulus. The second plug stops and maintains its sealing engagement with the casing once it arrives at the bottom of the casing.
When the casing string extends back to the drilling rig, the first and second plugs and cement are manually inserted into the casing at the drilling rig floor. Remotely set plugs are used when the well casing that is to be cemented does not extend back to the drilling rig floor. For example, a xe2x80x9cliner,xe2x80x9d which is a string of casing that hangs from the bottom of a previously installed larger diameter section of casing, does not extend back to the drilling rig floor. Subsea completions in offshore wells also involve strings of casing that do not extend back to the drilling rig.
Installing and cementing strings of casing that do not extend to the drilling rig is typically done by installing the casing string with a smaller diameter running string. If wiper plugs are employed in this process, they are carried on a running tool at the lower end of a small diameter string of drill pipe that extends from the drilling rig and connects to the top of the larger diameter casing string that is to be cemented. The drilling fluid and the cement slurry required to perform the cementing operation are initially pumped from the surface through the small diameter drill pipe, through circulating openings in the wiper plugs and into the casing. The plugs are xe2x80x9cremotely setxe2x80x9d from the rig floor using setting devices that are inserted into the string at the rig floor and pumped down to the plugs carried on the running tool. The cement slurry exiting the bottom of the casing string returns in the annulus to the point at which the casing string is hung off from the higher casing string or sub sea wellhead.
In a typical operation of remotely set wiper plugs carried at the end of a running tool on a drill string, a brass ball, or a weighted plastic ball or dart or other setting device is inserted into the drill string at the surface ahead of the cement slurry. The ball passes through the opening in the upper wiper plug and lands in and closes a smaller circulation opening in the lower plug. The resulting pressure increase releases the lower plug for movement through the casing. When sufficient cement has been pumped into the drill string and casing from the surface, a latch-down plug or seal dart is inserted into the drill string and pumped down to the upper wiper plug still secured to the running tool. Arrival of the latch-down plug at the upper plug closes the circulation opening and releases the upper plug for movement through the casing string. The upper plug is then pumped to the bottom of the casing to completely displace the cement slurry from the casing.
Remotely set wiper plugs are also employed in rig floor cementing assemblies that employ multipurpose tools that function as combination fillup tools and cementing tools. These combination tools, as described in U.S. Pat. No. 5,918,673, may include remotely releasable plugs in the surface operated assembly to eliminate the need for a separate plug container or other similar device at the rig floor for deploying the cementing plugs.
A common requirement of remotely set wiper plugs, including those used in the combination tool assembly, is the need for the plugs to accommodate circulation of fluids before they are released to travel through the casing string. The size of circulation openings is a major consideration in the design of the wiper plugs and their launching mechanisms.
In use, the materials and components of the wiper plug must withstand the pumping pressure differentials and the erosion experienced during different phases of the cementing procedure. Any sealing surface exposed to the flow of the cement slurry and drilling fluids is subject to erosion damage and possible failure, particularly when the seals are formed of plastic or other non-durable materials. Accordingly, substantial volumes of durable material are required in the construction of conventional wiper plug assemblies to meet the strength and erosion resistance requirements imposed on the assemblies before their release.
The increased strength and durability of the plugs are typically achieved at the expense of the size of the circulation openings through the plugs. Because of their relatively small circulation openings, remotely set wiper plugs carried in a combination tool or connected with the drill pipe can create a restricted flow passage for pumped fluids. These flow restrictions can increase the possibility of packing off and other problems and can limit pumping rates for the drilling fluids as well as the cement slurry.
The wiper plugs used in cementing must also be constructed of materials that may be easily drilled up or milled away at the end of the cementing operation. Because of this requirement, the use of high-strength metal is undesirable in the construction of the wiper plugs. The necessary strength and durability requirements are met in conventional wiper plugs by using larger volumes of soft metals and other easily removable materials. The required large volumes of material can require small passage openings that can contribute to the restriction of flow of fluids through the wiper plugs.
The requirement for relatively large volumes of soft structural metal or durable plastics within conventional, remotely actuated wiper plugs also renders the use of certain designs impractical within smaller internal diameter well casings. For example, in well casings having an internal diameter of 7xe2x80x3 or less, the volume of materials required to provide the support and release functions of a plug with a conventional design limit the fluid bypass opening so that desired pumping rates cannot be effectively obtained. The limited bypass openings also increase the likelihood of packing off the bypass and prematurely launching the plug.
Conventional, multi-plug assemblies employed in remotely launched systems typically require different designs for each wiper plug that is to be deployed within the well casing. Each of the different designs includes a large volume of the special material required for the structural support, sealing and latch release functions of the plugs. The total cost of employing conventional plugs includes the cost of the disposable materials incorporated into the plug and the requirement for separately dimensioned and designed plugs for each of the wiper plugs employed in the multi-plug assembly.
Gravity deployed balls used to launch a wiper plug may present certain operational difficulties with remotely operated plug launching systems. In particular, the ball""s position cannot be accurately determined as it falls through the drill string en route to the subsurface plug. The speed of travel of the ball through the drill pipe is affected by gravity and by the flow rate and viscosity of fluid being pumped through the drill string. The effect due to gravity can become particularly problematic when the drill pipe extends through non-vertical orientations common in directionally drilled wells.
An alternative to employing balls as the release activating mechanism for the plug is to employ pump-down darts that can be displaced through the drill pipe ahead of the well fluid or cement slurry being pumped down into the casing. The benefit of the dart release mechanism is that its position can be accurately determined by measuring the volume of fluid being pumped into the pipe behind the dart. The dart also functions as an effective wiping structure that cleans the internal surface of the drill pipe as it is being pumped down to the plug.
An additional benefit of pump-down darts is that the dart may be rapidly forced through the drill string and into position within the wiper plug deployment tool. By contrast, the time required for a ball to eventually reach the wiper plug system under the force of gravity assisted by cement or drilling fluid flow is unpredictable.
Remote cementing plug launching systems that can easily accommodate a ball are not necessarily capable of functioning with a pump-down dart because of the limited axial development of the launching system. When the system employs multiple plugs that are to be deployed from a single running tool, the axial spacing between the release mechanisms of the plugs can preclude the effective use of pump-down darts.
The present invention provides a cementing running tool with wiper plugs having large circulation openings that allow increased bypass flow of drilling fluids and cement slurries. The plugs are constructed using a minimal amount of material, which permits large circulation openings and also reduces the amount of material to be milled out at the completion of the cementing process. The running tool provides a central, thin-walled tubular mandrel and release sleeves constructed of high-strength steel that support the wiper plugs and protect them from erosion while they are attached to the tool.
A ball or dart may be used to release the wiper plugs from the mandrel. The steel mandrel and the ball or dart used to release the wiper plugs remain with the running tool, eliminating the problem of drilling up or milling those components. Easily drillable flapper valve closure devices carried on the wiper plugs close the circulation openings when the plugs are deployed from the running tool to eliminate the need for the releasing ball or dart to be sent to the bottom of the casing as is done in many prior art designs. The seal surfaces for the circulation openings are protected from erosion by the running tool. Multiple plugs run in series may be of similar design to reduce construction costs.
The system of the present invention employs high-strength steel in a relatively thin-walled mandrel and release mechanism of a retrievable running tool to support and subsequently deploy the cementing plug. The use of a retrievable thin-walled mandrel and release mechanism for supporting and providing the structure for release of the plug permits larger flow openings through the plug and, because the mandrel is reusable, reduces the total cost of employing the system.
An important feature of the present invention is the elimination of the use of a ball or dart that must remain in the wiper plug to act as the flow closure element for the deployed wiper plug. Because the ball and dart are retrieved with the mandrel, they may be constructed of any desired material without regard to their drillability. Moreover, retrieval of the ball or dart allows them to be reused to reduce costs.
A feature of the present invention is that the device used to close the flow opening in the wiper plug is an integral part of the plug assembly. A flapper gate secured to the plug body is automatically closed when the plug leaves the mandrel. During the pumping circulation phases of the cementing operation, the flapper gate and seat, which may be made of easily eroded material, are protected behind the release sleeve and mandrel preventing erosion of the sealing surfaces. By contrast, the seals in the retrievable parts of the running tool that are exposed to the pumped fluids in the system of the invention are constructed of a high-strength, erosion resistant material, such as high-strength steel.
Another important feature of the present invention is that substantially the entire cross-sectional seal area of the wiper plug is exposed to differential pressure during the pressure induced deployment of the plug from its supporting mandrel. Systems that apply a pressure differential over a more limited area produce a smaller separation force. The mounting of the wiper plugs to the mandrel is such that application of deployment pressure to the bottom plug does not stress the bypass provision for other higher plugs in the assembly.
A further feature of the present invention is that, in addition to protecting the seals and other vulnerable components of the wiper plugs, the thin-walled, high-strength, retrievable mandrel tube of the invention permits the use of plugs having a large central flow passage with a relatively small outside diameter for effective use in smaller casing sizes.
From the foregoing, it will be appreciated that an important object of the present invention is to provide cementing plugs that are run from a thin-walled, high-strength tubular mandrel and release structure that permits large bypass flow openings through the plugs to permit increased flow rates and protect the plugs from erosion during the pumping process.
A related object of the present invention is to provide a retrievable, high-strength, thin-walled running tool constructed of a high-strength steel that permits the use of plugs that have a relatively small outside diameter and a relatively large bypass opening to permit high flow rates of cement slurry and drilling fluids.
Yet another object of the present invention is to provide a cement plug deployment system and apparatus in which two or more plugs contained within the system have substantially the same design to minimize the cost of construction of the system.
Another object of the present invention is to provide a remotely operable cement plug system that can be activated by either balls or darts to selectively and separately deploy two or more wiper plugs from a retrievable running tool.
It is also an important object of the present invention to provide a running tool mandrel and release mechanism constructed of a high-strength steel to provide a thin-walled retention and isolation structure for remotely running one or more cement wiper plugs wherein the mandrel and release mechanism are retrievable parts of the running tool.
Another important object of the present invention is to provide the remotely operated cementing plug assembly of the present invention within a combination fillup tool and cementing tool disposed above the drilling rig floor.
The foregoing features, objects and advantages of the invention, as well as others, will become more fully appreciated and better understood by reference to the following drawings, specification and claims.